The symptoms can be due to inadequate mobilization and strengthening post surgery.
If you have a detailed exercise plan prescribed by Physio for post op rehab do it properly for few weeks.
Otherwisr consult a Physiotherapy specialized in ACL rehab
Gud luck.

Greetings sorry about your pain. Rather than following advices given just get a physio opinion about the knee status and if possible send me your knee photos in standing both front and back. Will guide you further.

Consult a good physiotherapist in your area. You need a conditioning program. The intensity duration and frequency of the exercises matters and also don't forget you need to maintain the strength achieved through exercises.

Avoid sitting Cross legged.
Avoid Squatting-
Quadriceps Exercises- Lie straight, make a towel role and put it under the knee, press the keen against the role, hold it for 20 secs. Repeat 20 times twice a day. This will help relieve some pain.
Core Strengthening Exercise- Straight Leg Raised With Toes Turned Outward, repeat 10 times, twice a day.
Hams Stretching- lie straight, take the leg up, pull the feet towards yourself, with a elastic tube or normal belt. repeat 10 times, twice a day.
Sports Taping- stretch the tape from both ends and apply on the affected area
Contrast Fomentation (Hot and Cold).

Physiotherapy is needed.
Knee exercises - static quadriceps : lie flat on back and press your knee. You can use pillow and knee and press.
Lie flat and slowly flod your knee.
In sitting position : lift your leg up
You can search these exercises through Internet - static and dynamic quadriceps or isometric and isotonic exercises of knee.
Static cycle : cycling for 15 mins in 2 sessions daily.
Make it tight slowly after every week. This strengthens your muscle a lot.